1 Materials Beyond Silicon By Uma Aghoram Moores

1 Materials Beyond Silicon By Uma Aghoram

Moore’s Law • MSI LSI VLSI • Moore’s Law states that the number of transistors on a chip doubles about every two years. • At each stage of scaling fundamental limits are being reached TI Fellows Forum

SCALING LIMITS • Lithographic Limits – XRay and E-beam in future • Short channel effects – Vt roll off – DIBL • Oxide thickness scaling reaching a few atomic layers of silicon – Large leakage currents – Higher power dissipation TI Fellows Forum

Exponential Forever ? 130 nm���� 90 nm���� 60 nm���� 45 nm���� 30 nm���� ? TI Fellows Forum

KEEPING MOORE’S LAW ALIVE • Strained Silicon • Novel Device structures • High K dielectrics • Carbon Nanotubes, SET • Alternate channel materials TI Fellows Forum

REPLACING SILICON • Cost effective • Easy incorporation into existing technology • Reliable oxide • Performance enhancement TI Fellows Forum

Germanium ADVANTAGES • Has high electron and hole mobility as compared to silicon – High speed Material mn mh Silicon 1450 500 Germanium 3900 2270 • high-K dielectrics • Good compatibility with III-V materials DISADVANTAGES • No reliable native oxide • Small Bandgap may lead to larger tunneling currents TI Fellows Forum

Gallium Arsenide and other III-V materials ADVANTAGES • Has 6 X larger electron mobility as compared to silicon – High speed • • Material mn mh Silicon 1450 500 Ga. As 9200 400 Have excellent Optoelectronic properties High resistance to radiation damage High flexibility to forming alloys Heat resistant TI Fellows Forum

Gallium Arsenide and other III-V materials DISADVANTAGES • No reliable native oxide • Composite nature leads to high defect density. Unstable Vt • More expensive to manufacture pure Ga. As to meet industry standard • Lower thermal conductivity than Silicon • More fragile • More quantum effects • Small recombination time results in poor performance of Ga. As BJTs. • High leakage current in narrow gap III-V materials TI Fellows Forum

Where do we stand? Germanium • Strained Si Mosfets with Si. Ge layers have been manufactured • Dual channel heterostructure MOSFETS are being researched. Gallium Arsenide: • Bell Labs and Freescale recently reported that they have grown high quality Ga 2 O 3 on Ga. As with low interface states and achieved enhancement and depletion mode MOSFETs. • HBT’s, HEMT, Power transistor TI Fellows Forum

Toy Problem • Position of charge centroid in a MOSCAP for three different materials namely: – Silicon – Germanium – Gallium Arsenide • The position where the charge peaks is of relevance as it determines additional thickness of the dielectric and effectiveness of gate control of device TI Fellows Forum

Assumptions • • One dimensional problem One band effective mass Hamiltonian Voltage on gate directly applies to the channel 20 A of oxide and varying well widths Wave functions are strongly excluded form the oxide Lattice spacing and dimension varies with material (100) surface Inversion charge density was a constant at 1 e 13/cm 2 Vg X insulator source channel drain insulator Z 0 TI Fellows Forum

PROCEDURE • 1 -D Schrodinger Poisson Solver [Hz+U]fm= em fm U(r) TI Fellows Forum n(z)

Procedure • From Schrödinger part of the solution we get the eigen values and eigen functions. • We can then calculate the electron density n(z) • Using the Poisson's equation we then solve for the self consistent potential, and use this new potential in the Schrödinger's equation and thus solve self consistently. • For the Hamiltonian the mass to be used is the out of plane effective mass and for the density of states expression use the DOS mass. TI Fellows Forum

Results Ninv=1 e 13/cm 2 Si Ge Ga. As In. Sb TI Fellows Forum

CONCLUSIONS • As the mass reduces the quantum confinement effects are more significant and peaking of the charge concentration takes place deeper in the substrate. • This means that the effective gate capacitance is smaller in Ge and Ga. As as compared to silicon • Also there is better gate control of channel in silicon devices as compared to Ga. As and Ge • Higher voltages are required by III-V materials to reach the same inversion charge density when compared to silicon • Lower transconductance in materials with lower mass. TI Fellows Forum

Existing Results TI Fellows Forum

Current Research Germanium • The main supporters of Germanium are Sematech, IBM, Umicore and Soitech. A lot of research is also being carried out in this field by IMEC in partnership with Umicore and Soitech. Ga. As • Intel is one of the main supporters of Ga. As • Quintec researches in this area TI Fellows Forum

References and Acknowledgement • Quantum Transport – Atom to transistor Supriyo Dutta Cambridge university press • Physics of Strain Effects in Semiconductors and MOSFETs Y. Sun, S. E. Thompson, and T. Nishida, awaiting publication • Gallium Arsenide –Ga. As as a semiconductor, its turbulent past, shaky present and promising but distant future Aseem Srivastava IEEE 1989 • http: //www. edn. com/article/CA 6314526. html? ref=nbra • http: //www. investorrelations. umicore. com/en/press. Releases/2003/germanium_E. p df • Fundamentals of modern Vlsi devices Taur and Ning • http: //mems. caltech. edu/courses/EE 40%20 Web%20 Files/effective%20 mass%20 exp lanation. pdf • Indium Phosphide and related materials, 2005 International conference on • www. intel. com • ftp: //download. intel. com/research/silicon/Gordon_Moore_ISSCC_021003. pdf • www. compoundsemiconductor. net/articles/news/10/1/25/1 TI Fellows Forum

20 Thank you!! Questions? ? ?

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